When providing emergency cardiac patient care, it is essential to generate the patient's electro-cardio graph (ECG) quickly and accurately for proper diagnosis and successful treatment. A typical ECG signal measuring system 10 is shown in FIG. 1. In this example, ECG signal measuring system 10 is part of diagnostic-quality monitor/defibrillator 12. To measure the ECG signal of a patient 14, ECG signal measuring system 10 is coupled to patient 14 through electrodes 15 and 16 and a signal acquisition and digitizing circuit 17. Signal acquisition and digitizing circuit 17 is a standard circuit configured to receive the analog ECG signals from the electrodes and convert this signal into a digital signal. ECG signal measuring system also includes a baseline wander filter (BWF) 18.
Large amplitude, low-frequency, non-physiological signals, commonly referred to as baseline wander, are generally sensed along with the patient's ECG. There are several sources of baseline wander including; DC bias currents, patient movement and changing patient impedance. Several of the sources of baseline wander are described further below.
ECG signal measuring systems used for emergency medical applications typically use a DC bias current to detect disconnected electrode leads. This current interacts with the patient's impedance to cause a relatively high amplitude but low frequency signal that is superimposed on the relatively low voltage ECG signal when electrodes 15 and 16 are initially applied to patient 14. For convenience, this signal is referred to herein as the bias current signal. This bias current signal is illustrated in FIG. 2 by a curve 20. As can be seen in FIG. 2, an initial portion 21 of curve 20 has a relatively large rate of change. The bias current signal eventually begins to stabilize, as indicated by a portion 23 of curve 20. The bias current signal results in a significant rate of change of the combined input signal (i.e., the baseline wander combined with the patient ECG signal) during the initial period. This rate of change of the combined input signal is referred to herein as the slew rate. When the bias current signal eventually starts to stabilize, the slew rate of the combined input signal is reduced.
Baseline wander can also be caused by movement of patient 14 or electrodes 15 or 16 that disturbs the electrical connection of electrodes 15 and 16 to patient 14. This movement can result in a significant change in the impedance presented to ECG signal measuring system 10. This change in impedance can result in a change in the bias current signal, which results in a change in the level of the combined input signal.
Baseline wander can also be caused by interaction of the bias current with changing patient impedance caused by the electrodes forming a better electrical connection to the patient over time.
Generally, the baseline wander is estimated and the estimate is subtracted from the combined signal before displaying the output ECG. One conventional system for estimating baseline wander is illustrated in FIG. 3. FIG. 3 is a functional block diagram illustrative of conventional BWF 18 (FIG. 1) for use in a digital ECG measuring system. BWF 18 includes an infinite impulse response (IIR) baseline wander estimator (BWE) 30, an adder 32 and a delay circuit 34. BWE 30 is connected to receive input ECG samples, denoted ECG.sub.i (n), and output an average of the samples that represents the estimated baseline wander samples, denoted BW(n). Delay circuit 34 also receives ECG.sub.i (n), and delays each sample by the delay of BWE 30. In this way, each BW(n) sample is synchronized with the corresponding ECG.sub.i (n) sample. Adder 32 receives each estimated baseline wander BW(n) sample and subtracts it from the corresponding input ECG sample ECG.sub.i (n), thereby generating output ECG samples, ECG.sub.o (n).
One problem with using a BWE based on an IIR filter is that, in some conventional systems, the IIR filter normally provides non-linear phase response. To linearize the phase response in these conventional systems, the data must be filtered, then the output samples must be reversed in order, and filtered through the IIR filter again. This "forward-backward" operation adds delay to the process, and requires that the data be processed in batches. Thus, each filtered "batch" must then be appended to the previous filtered "batch", which, undesirably, tends to cause discontinuities in the output ECG. Other conventional systems that use IIR filters to remove baseline wander employ a slew-rate limiter to clip portions of the input ECG signal. The slew-rate limiter adds complexity and cost to the system. Generally, these conventional systems use IIR filters to avoid the relatively high computation load of conventional finite impulse response (FIR) systems. Thus, there is a need for a BWE that is computationally efficient, has linear phase response, and does not require an additional circuitry such as a slew-rate limiter.